提供判断是否为素数的三种常用方法，其基本思想是用该数分别处以比其小的所有数，若只能1和其本身除尽则为素数；否则不是-Provided for the prime numbers to determine whether three commonly used methods, the basic idea is fine with the respectively smaller than the number of all, if only divisible by 1 and compared with its own prime number　otherwise not